1. Field of the Invention
The present invention relates to an improved image projection system that uses a filter (holographic, dielectric, or diffraction grating) that is subpixelated to pass selected colors of light (preferably one color of light for each subpixel) to a micromirror device that has an array of mirrors corresponding in number the number of subpixels of the dielectric filter or to a liquid crystal display device. Even more particularly the present invention relates to a projection system having a subpixelated dielectric filter that each pass a selected color light (reflecting others) to a micromirror or a LCD device to define a parallel color device that simultaneously transmits multiple rays of light of selected colors to an image screen rather than sequential transmission of the color rays.
2. Description of the Related Art
Projection systems are used to display images on large surfaces, such as movie or television screens. For example, in a front projection system, an image beam is projected from an image source onto the front side of a reflection-type angle transforming screen, which then reflects the light toward a viewer positioned in front of the screen. In a rear projection system, the image beam is projected onto the rear side of a transmission-type angle transforming screen and transmitted toward a viewer located in front of the screen.
In prior co-pending U.S. patent application Ser. No. 08/581,108, entitled "Projecting Images," to Knox, filed Dec. 29, 1995, there is disclosed a method of displaying an optical image by projecting the image along an optical path and at an optical device interposed across the optical path, at one time reflecting the image from the optical device and at a different time permitting the image to pass through the optical device to be displayed.
One requirement for such an image projection systems is that a linearly polarized image be presented so that the image first is reflected off the screen surface and the second time is transmitted through the screen surface after a 90.degree. rotation of the polarization. Systems have been previously disclosed that attempt to provide such a polarized image. For example, in U.S. Pat. No. 5,453,859, a system is shown that uses a polarization beam splitter along with a dichroic "X-cube" to create a color polarized image.
Displaytech, Inc., in a 6-page technical disclosure entitled "FLC/VLSI Display Technology" and dated 1 Dec. 1995; Parfenov, et al., in "Advanced optical schemes with liquid crystal image converters for display applications," SPIE Proceedings, Volume 2650, pages 173-179 (29-31 Jan. 1996); and Baur, et al., in "High performance liquid crystal device suitable for projection display," SPIE Proceedings, Volume 2650, pages 226-228 (29-31 Jan. 1996), disclose general background information on the use of liquid crystal devices to process video images. The disclosures referred to in this paragraph are hereby incorporated herein by reference.
One type of image generating reflector that can be used in a projection system is a deformable micromirror device. Such as device has mechanical micromirrors that are created in a semi-conductor device through semiconductor techniques. These are known to the art and are described, for example, in U.S. Pat. No. 5,083,857 issued to Larry J. Hornbeck entitled "Multi-Level Deformable Mirror Device." That patent is hereby incorporated by reference and further discloses the details of micromirror devices.
Micromirror devices, however, are not color selected. That is, they typically are color sequential when used in display systems. This is because they simply reflect whatever light hits them, they have no need for any masking technologies.
A variety of "image engines" are used for image projection systems. One type of engine is a "sequential color" engine, in which a red image, green image, blue image, or other three contrasting color images, are sequentially transmitted at high speed. The perceptual result is a color image that is a combination of the three. Such images allow a single monochrome image source to be used, with appropriate color filtering.
A problem with such sequential engines, however, is that they typically must operate at a high rate of speed. This typically limits such sequential image engine to on/off liquid crystal displays, or other on/off systems, instead of analog systems. (Analog systems are typically too slow.) Even then, it is difficult to create a large number of colors because of timing considerations.
An alternative is to provide color pixels. A problem with color pixels, however, is typically they have used absorptive color filters. For example, on a color active matrix liquid crystal display used on a laptop computer, each pixel is colored red, green, or blue, and transmits its selected color, but absorbs the remaining frequencies. This is fine for a laptop display, but can cause problems with the very high intensity light sources used in projection systems because of heating.
Further, when colorized pixels are used, they typically absorb the unused light, thus reducing efficiency. Because effectively two-thirds of the light for each pixel is absorbed rather than transmitted, only one-third of the original light source is actually used. In image projection systems, this requires even brighter light sources to achieve the same luminance.
Subpixelated systems also have had the problem that color purity is to be degraded by crossover from adjacent subpixels. That is, some of the light destined for one pixel may bleed into the adjacent pixel, degrading the color purity.